01In plain English
A model can only consider a limited amount of text at a time, measured in tokens. Everything counts toward it: the prompt, any files you attach and earlier messages. When a conversation runs past the limit, older parts drop out.
02Why it matters when you are choosing
A bigger context window means a tool can read a long contract or a whole codebase in one go. A small one forces it to work in pieces and forget earlier detail.
03What to check
Ask the vendor
Check the context size on your plan, since tools often give the largest window only on higher tiers.